This invention relates to a thin-type aluminum solid electrolytic capacitor using a flat-plate aluminum foil and a fabrication method thereof and, in particular, relates to an ultra-miniature, stacked large-capacity, and low-impedance thin-type aluminum solid electrolytic capacitor and a fabrication method thereof. This invention relates to a structure and a fabrication method of a stacked thin-type aluminum solid electrolytic capacitor using electrolytic capacitor aluminum foils and having low impedance characteristics.
Following miniaturization, speed-up, and digitization of electronic devices in recent years, there has been a strong demand for miniature large-capacity capacitors having excellent high-frequency characteristics of impedance also in the field of capacitors.
Capacitors that are used in a high frequency region have conventionally been mainly multilayer ceramic capacitors which, however, cannot satisfy need for reduction in size, increase in capacity, and reduction in impedance.
As large-capacity capacitors, there are electrolytic capacitors such as conventional aluminum electrolytic capacitors and tantalum solid electrolytic capacitors. However, electrolyte solutions or electrolytes (manganese dioxide etc.) used in those capacitors each have a high resistivity value (1Ω·cm to 100Ω·cm) and therefore it has been difficult to obtain a capacitor having a sufficiently low impedance in a high frequency region.
In recent years, however, there have been developed solid electrolytic capacitors each using a conductive polymer such as polypyrrole or polythiophen as a solid electrolyte. As compared with the conventional solid electrolyte in the form of a metal oxide semiconductor such as manganese dioxide, the solid electrolyte in the form of the conductive polymer has a smaller resistivity value (0.01Ω·cm to 0.1Ω·cm). An impedance value Z in a high frequency region is proportional to a resistivity value ρ of a used electrolyte, that is, Z∝ρ. Therefore, the solid electrolytic capacitor using the conductive polymer having the small resistivity value as the solid electrolyte can suppress the impedance value in the high frequency region to a lower value and thus those capacitors are now widely used.
As one example of an aluminum solid electrolytic capacitor using a conductive polymer as a solid electrolyte, a flat-plate element structure will be described. An anodic oxide coating layer is formed on the surfaces of a belt-shaped surface-roughened (etched) aluminum foil and a resist band made of an insulating resin such as epoxy resin is formed at a predetermined portion for defining an anode portion and a cathode portion. Thereafter, conductive polymer film is formed at a predetermined portion and then a graphite layer and a silver paste layer are formed on the conductive polymer film in the order named, thereby forming the cathode portion. Thereafter, the cathode portion and an external cathode terminal are connected together by the use of a silver paste. Since the anode portion defined by the resist band is in the form of the aluminum foil which is unsolderable, a solderable metal plate is electrically connected thereto by ultrasonic welding, electric resistance welding, laser welding, or the like.
On the other hand, in recent years, in order to achieve a large capacity and low impedance characteristics with a limited floor area, there is a stacked capacitor in which a plurality of aluminum solid electrolytic capacitor elements each using a conductive polymer as a solid electrolyte are stacked together, cathodes are bonded together by a conductive paste, and further, anode terminals are pierced and a conductive paste is applied to such a portion, thereby achieving electrical connection therebetween. Such example is disclosed in Japanese Unexamined Patent Application Publication (JP-A) No. 2004-158577, paragraphs 0011 to 0021 and FIG. 1.
Further, in order to achieve reduction in impedance in a high frequency region, there is also an aluminum solid electrolytic capacitor having a three-terminal structure in which both ends of a surface-roughened flat-plate aluminum base member of a certain size serve as anodes, a cathode with an electrolyte is provided at the center, and an insulating layer is formed between the cathode and each of the anodes. Japanese Unexamined Patent Application Publication (JP-A) No. 2004-15706, paragraphs 0023 to 0025 and FIG. 1 discloses such a tree-terminal structure capacitor.
In the stacked aluminum solid electrolytic capacitor, the contact resistance is generated due to connections between the anode terminals and between the cathode terminals that are formed by stacking the single-plate elements and, as the number of the stacked elements increases, the influence of resistivity of the conductive paste used for the connection increases. Therefore, it has been difficult to achieve reduction in impedance of the stacked aluminum solid electrolytic capacitor.
Further, in the conventional thin-type aluminum solid electrolytic capacitor using the flat-plate aluminum foil, there is a problem that as the element floor area decreases, the ratio of the cathode portion occupying the element floor area decreases. For example, in the case of a stacked aluminum solid electrolytic capacitor having an element floor of W(width)×L(length)=4.3×7.3(mm2) or less, the ratio between an effective floor area (an area of a cathode portion occupying the element floor) and an element floor area, i.e. the effective floor area/element floor area, becomes about 60% or less. Further, when the floor area is W(width)×L(length)=2.8×3.5(mm2) or W(width)×L(length)=1.6×3.2(mm2), the effective floor area/element floor area becomes about 50% or less. Therefore, the conventional structure is unsuitable for forming a miniature large-capacity stacked solid electrolytic capacitor.